The term “computer system” today applies to a wide variety of devices. Mainframe computers, personal computers, personal digital assistants, and cellular telephones are but a few examples. In computer systems, a graphics controller (“GC”) is commonly employed to couple a CPU to a display device, such as a CRT or an LCD. A GC is typically used to perform certain special purpose functions related to the display of text and images so that the CPU is not required to perform such functions. Commonly, a GC is capable of performing image transformation operations, such as cropping and scaling an image. In addition, a GC may have a data compressor/decompressor (“CODEC”) for sending and receiving compressed data.
When image data is transmitted between computer systems, such as over the Internet, a large amount of transmission capacity or “bandwidth” is required. For this reason, the time required to transfer image data can be quite long. In addition, the transfer of image data creates congestion in the network, causing the transmission of other data to slow. One solution to this problem is to compress the data before transmitting it. A popular method for compressing still images was developed by the Joint Photographic Experts Group (“JPEG”) and files compressed according to the method are referred to as “JPEG files” or as being in the “JPEG format.” The JPEG standard includes variations and versions and, for ease of explanation, this acronym is used herein in a general sense to encompass all such variations and versions. Before it is compressed into a JPEG file, the image data must first be transformed and divided into groups or “blocks.” In addition, the blocks of image data are arranged in an alternating or “interleaved” sequence before being compressed. When a computer system receives a JPEG file, it must decompress the file before it can be displayed. In addition, because the image data is in a block-interleaved sequence after decompression (referred to as “JPEG decoded block-interleaved data”), regrouping as well as transformation operations are required. In a computer system with a GC having a CODEC, the GC performs data compression and decompression, as well as the other required operations.
The compression of image data greatly improves the speed with which networks can transfer data. Moreover, the use of a GC to handle compression and decompression tasks frees the CPU to perform other work. When the image data is decompressed by the GC, however, the quantity of data increases dramatically. Further, the amount of bandwidth required to transfer image data between modules within a GC becomes very large. The decompressed image data is typically stored and read from a memory before it is displayed, and accordingly, these operations consume an amount of bandwidth that is substantial. Thus, the processing of decompressed image data imposes a significant load on the GC, which causes the power consumed by the GC to increase. In addition, the high memory bandwidth requirement creates a memory access bottleneck that makes memory access time for other devices lengthen. In particular, it inhibits the CPU's ability to access memory in a timely manner. A method or apparatus capable of reducing the amount of memory bandwidth a GC requires to transfer decompressed image data to and from memory would clearly be desirable.
Accordingly, there is a need for a method and apparatus for reducing the bandwidth required to transmit image data, and more particularly to a method and apparatus for reducing the bandwidth required to transmit JPEG decoded block interleaved data.